mesh-mail: say mail events on its own stream MAIL, granted per account by grants (hq ADR 0363)

ADR 0359 asked for a stream of its own and a bus grant from the module's
grants; ADR 0363 built both. The events leave the shared EVENTS stream, and
one family pattern per change replaces the literal per-account emits, so a
new account changes no manifest.

modules/mesh-mail/module.json

@@ -31,10 +31,20 @@
     "mail_read",
     "mail_health"
   ],
-  "emits": [
-    "mail.mesh.message.added",
-    "mail.mesh.message.changed",
-    "mail.mesh.message.removed"
+  "streams": [
+    {
+      "name": "mail",
+      "events": [
+        "mail.*.message.added",
+        "mail.*.message.changed",
+        "mail.*.message.removed"
+      ],
+      "max-age-days": 7,
+      "grant": {
+        "config": "grants",
+        "events": "mail.{key}.**"
+      }
+    }
   ],
   "resources": [
     {